The present invention relates to an image coding apparatus and an image decoding apparatus for coding and decoding digital images, and also relates to recording media in which image coding/decoding programs are stored.
One of known conventional image coding systems requiring reversibility is the MMR (Modified Modified Read) system which is used in facsimile applications. However, since the MMR system is a system in which processing is performed for the entire one frame input, it is difficult to maintain consistency with another coding system adopted in ITU-T H.261 and ISO/IEC MPEG, etc. in which one frame is divided into a plurality of blocks and processing is performed in block units. Thus, the MMMR (Modified MMR) that is an improved version of the MMR system is adopted as an verification model of MPEG4 (ISO/IECJTC/SC29/WG11 N1277, July 1996).
An image coding system which divides a one-frame image into two-dimensional blocks consisting of a plurality of pixels and codes the image for each block, performs coding by dividing data into two parts: one is coding mode data which indicates whether or not block movement is corrected for each block, presence or absence of a quadrature conversion coefficient and whether or not a block has significance in the case that the input image is a form image which indicates the form and synthesis ratio of the object, etc., and the method for coding pixel values within the block, and the other is coding data of the pixel values within the block.
However, the coding method that performs coding by dividing the above image block into the coding mode data that indicates the coding mode of said block and the data concerning the pixel values within said block has the problem that the percentage of the block mode data in the created coded signal relatively increases, deteriorating its coding efficiency in the case that blocks without movement information, quadrature conversion coefficient or significant form, etc. increases within one frame.
The objective of the present invention is to provide an image coding apparatus and image decoding apparatus that will prevent the coding efficiency from drastically deteriorating even for those images that contain numerous blocks without movement information, quadrature conversion coefficient or significant form, etc. within one frame.
The image coding apparatus of the first invention is an image coding apparatus that, when coding the mode data, predicts the coding mode of a block to be coded from the mode data of already coded peripheral blocks and switches a code word table that specifies code words according to the hitting ratio of prediction.
The image decoding apparatus of the second invention is an image decoding apparatus that, when decoding the mode data, predicts the coding mode of a block to be decoded from the mode data of already decoded peripheral blocks and switches a code word table that specifies code words according to the hitting ratio of prediction.
The first and second inventions can improve the coding efficiency by shortening the code word length of a coding mode with high hitting ratio.
The image coding apparatus of a third invention is an image coding apparatus that collectively codes the coding mode data corresponding to a plurality of blocks for a digital image which has been divided into those blocks.
The image decoding apparatus of the fourth invention is an image decoding apparatus that collectively decodes code words regarding the mode data corresponding to a plurality of blocks.
The third and fourth inventions can easily perform coding/decoding using a correlation between blocks of the mode data, thus improving coding efficiency.
The image coding apparatus of the fifth invention is an image coding apparatus that, when coding the mode data, predicts the coding mode of a block to be coded from the mode data of already coded peripheral blocks and switches a probability model that specifies appearance probability according to the hitting ratio of prediction.
The image decoding apparatus of the sixth invention is an image decoding apparatus that, when performing arithmetic decoding of mode data, predicts the coding mode of a block to be decoded from the mode data of already decoded peripheral blocks and switches a probability model that specifies appearance probability according to the hitting ratio of prediction.
The fifth and sixth inventions can perform arithmetic coding/decoding by predicting a coding mode and adaptively switching to a probability model for which a coding mode with high appearance probability is set, thus improving coding efficiency.
The recording medium of the seventh invention is a computer recording medium that stores the program data that implements at least one of the image coding apparatus/image decoding systems that relate to the first to sixth inventions.